1. Field
The present invention relates generally to communications, and more specifically to a novel and improved method and apparatus for chip rate processing.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other modulation techniques. A CDMA system provides certain advantages over other types of systems, including increased system capacity.
A CDMA system may be designed to support one or more CDMA standards such as (1) the “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), (2) the “TIA/EIA-98-C Recommended Minimum Standard for Dual-Mode Wideband Spread Spectrum Cellular Mobile Station” (the IS-98 standard), (3) the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (4) the standard offered by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in a set of documents including “TR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems,” the “C.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,” and the “C.S0024 cdma2000 High Rate Packet Data Air Interface Specification” (the cdma2000 standard), and (5) some other standards. These standards are incorporated herein by reference. A system that implements the High Rate Packet Data specification of the cdma2000 standard is referred to herein as a high data rate (HDR) system. Proposed wireless systems also provide a combination of HDR and low data rate services (such as voice and fax services) using a single air interface.
Pseudorandom noise (PN) sequences are commonly used in CDMA systems for modulation of transmitted data, including transmitted pilot signals. CDMA receivers commonly employ RAKE receivers. A rake receiver is typically made up of one or more searchers for locating direct and multipath pilots from neighboring base stations, and two or more fingers for receiving and combining information signals from those base stations.
In general, the performance of any CDMA system is enhanced as more fingers are added to receivers in order to process a greater number of multipath signals from one or many base stations. This is particularly true as the chip rate used to spread incoming signals increases, as more components of the multipath signal are then distinguishable at the receiver. The W-CDMA standard describes such a system where the ability to demodulate a high number of signal components is desirable.
CDMA demodulators often include dedicated hardware, known as finger front ends, to process the relatively higher chip rate data that is received. Often a DSP or other processor is deployed to receive symbol rate data from the finger front end to further demodulate the symbols. One way to enhance the performance of any CDMA system, or to meet specifications for a higher chip rate system, is to replicate the hardware of one finger for as many fingers as are required. While this technique has been used with success in the past, as finger requirements grow, the resultant hardware requirements can become prohibitively expensive. An alternate technique is to provide a general purpose DSP capable of performing chip rate processing, although this too can be expensive in hardware and may require impractical clock rates and associated power drain to implement in a high speed system with a large number of channels to demodulate.
There is therefore a need in the art for a finger front end capable of processing a large number of channels delivered at high chip rate in a high throughput, hardware efficient manner.